Inner focus lens system, interchangeable lens apparatus and camera system

Abstract

An inner focus lens system, in order from an object side, comprising: a first lens unit including a most-object-side negative first lens element; a positive second lens unit; and a negative third lens unit, wherein an aperture diaphragm is included, the first lens unit, the third lens unit, and the aperture diaphragm are fixed relative to an image surface in focusing, the second lens unit moves relative to the image surface in the focusing, and the conditions: BF/Y<1.7 and TH/f>1.6 (BF: distance from an apex of an image-side surface of a most-image-side lens element to the image surface; Y: maximum image height; TH: distance from an apex of an object-side surface of a most-object-side lens element to the apex of the image-side surface of the most-image-side lens element; f: focal length of the lens system) are satisfied.

What is claimed is: 
     
       1. An inner focus lens system having a plurality of lens units each being composed of at least one lens element, the inner focus lens system therein comprising eight or more lens elements, wherein:
 in the lens system being focused from an infinity in-focus condition to a close-object in-focus condition, a lens unit located closest to an object side of the lens system is, with respect to an image surface for the lens system, fixed; 
 the lens unit located closest to the object side of the lens system includes a first lens element located closest to the object side, and a second lens element located, relative to the first lens element, on an image side of the lens system; and 
 the lens system satisfies following conditions (1) and (3):
     BF/Y< 1.7  (1)
 
   0<|( r   2a   +r   1b )/( r   2a   −r   1b )|<1  (3)
 
 
 where 
 BF is distance from an apex of an image side surface of a lens element located closest to the image side in the lens system, to the image surface, 
 Y is maximum image height, 
 r 1b  is radius of curvature of an image-side surface of the first lens element, and 
 r 2a  is radius of curvature of an object-side surface of the second lens element. 
 
     
     
       2. The inner focus lens system as claimed in  claim 1 , wherein:
 the inner focus lens system includes
 a second lens unit located on the image side relative to the lens unit located closest to the object side in the lens system, and 
 a third lens unit located on the image side relative to the second lens unit; 
 
 and the lens system satisfies following condition (4):
   −2.5< f   2   /f   3 <−0.25  (4)
 
 
 where 
 f 2  is focal length of the second lens unit, and 
 f 3  is focal length of the third lens unit. 
 
     
     
       3. The inner focus lens system as claimed in  claim 1 , further satisfying the following condition (5):
   0.23< D   max   /Y< 1.3  (5)
 
 where 
 D max  is maximum value among air spaces between the lens elements constituting the lens system in an infinity in-focus condition, and 
 Y is maximum image height. 
 
     
     
       4. The inner focus lens system as claimed in  claim 1 , further satisfying the following condition (6):
     nd   p >1.79  (6)
 
 where 
 nd p  is refractive index to the d-line of the lens element located closest to the object side in the lens system, among lens elements having positive optical power. 
 
     
     
       5. The inner focus lens system as claimed in  claim 1 , further satisfying the following condition (7):
   ( r   1b   +r   1a )/( r   1b   −r   1a )<−1  (7)
 
 where 
 r 1a  is radius of curvature of an object-side surface of the first lens element, and 
 r 1b  is radius of curvature of the image-side surface of the first lens element. 
 
     
     
       6. The inner focus lens system as claimed in  claim 1 , further satisfying the following condition (8):
   | f   1   /f|< 3.0  (8)
 
 where 
 f 1  is focal length of the lens unit located closest to the object side in the lens system, and 
 f is focal length of the lens system in the infinity in-focus condition. 
 
     
     
       7. The inner focus lens system as claimed in  claim 1 , further satisfying the following condition (9):
     vd   12 >60  (9)
 
 where 
 vd 12  is the larger value between an Abbe number to the d-line of the first lens element, and an Abbe number to the d-line of the second lens element. 
 
     
     
       8. The inner focus lens system as claimed in  claim 1 , further comprising an aperture diaphragm, wherein the lens system further satisfies the following condition (10):
   0.4< Ts/L< 0.8  (10)
 
 where 
 Ts is axial distance from the aperture diaphragm to the image surface, and 
 L is overall length of lens system, being axial distance from an object-side surface of the first lens element to the image surface. 
 
     
     
       9. The inner focus lens system as claimed in  claim 1 , wherein:
 the inner focus lens system includes a second lens unit located on the image side relative to the lens unit located closest to the object side of the lens system; and 
 the inner focus lens system is provided with an aperture diaphragm located on the object side relative to the second lens unit. 
 
     
     
       10. The inner focus lens system as claimed in  claim 1 , wherein
 the inner focus lens system includes a second lens unit located on the image side relative to the lens unit located closest to the object side in the lens system; and 
 a lens element located closest to the object side in the second lens unit is a lens element that has a convex surface facing the image side, and has negative optical power. 
 
     
     
       11. The inner focus lens system as claimed in  claim 1 , wherein:
 the inner focus lens system includes
 a second lens unit located on the image side relative to the lens unit located closest to the object side in the lens system, and 
 a third lens unit located on the image side relative to the second lens unit; and 
 
 a lens element located closest to the image side in the third lens unit has negative optical power. 
 
     
     
       12. An interchangeable lens apparatus comprising:
 an inner focus lens system as claimed in  claim 1 ; and 
 a lens mount section which is connectable to a camera body including an image sensor for receiving an optical image formed by the inner focus lens system and converting the optical image into an electric image signal. 
 
     
     
       13. A camera system comprising:
 an interchangeable lens apparatus including an inner focus lens system as claimed in  claim 1 ; and 
 a camera body detachably connected to the interchangeable lens apparatus via a camera mount section, and including an image sensor for receiving an optical image formed by the inner focus lens system and converting the optical image into an electric image signal.